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Unseen Saturn

Saturn's aurora forms a faint blue-green ring around the planet's south pole in this false-color image from the Cassini spacecraft. The image was compiled from infrared observations at three wavelengths. The blue shows reflected sunlight, while the red shows heat from the planet itself. These red zones have thinner layers of cloud cover, allowing us to see deeper into the planet's thick atmosphere. The infrared observations reveal details about Saturn that visible light cannot, adding to our knowledge of the planet. [NASA/JPL/University of Arizona/University of Leicester]

Saturn is sliding down the southwestern sky as we head into the last month of summer. It looks like a bright golden star, and is far to the upper left of brilliant Venus as night falls this evening.

We see Saturn because it reflects sunlight. Saturn is the second-largest planet in the solar system, so it’s a big target. And it’s topped by clouds that reflect much of the visible light that strikes them back out into space.

But there’s a lot that our eyes miss. Saturn and its rings reflect other wavelengths of light, such as infrared and ultraviolet. Spitzer Space Telescope used the infrared to discover a wide but faint ring that’s far outside the visible rings.

Saturn also produces a lot of infrared. The planet’s gravity squeezes it tightly, heating Saturn’s interior to temperatures that are hotter than the surface of the Sun. And a “rain” of liquid helium may add to that heat. The heat escapes into space as infrared energy. In fact, Saturn actually radiates twice as much energy into space as it receives from the Sun. More about infrared astronomy tomorrow.

Saturn is also a weak source of radio waves. They, too, are born deep inside the planet. Different layers of the planet’s interior spin at different rates. That acts like an electric motor, which produces a strong magnetic field. The field traps charged particles from the Sun and elsewhere. The particles vibrate, releasing radio waves that beam into space — adding to Saturn’s unseen glow.

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